Classic Electrically Small Antennas Versus in/On-Body Antennas: Similarities and Differences

Electrically small antennas (ESAs) have been discussed since the early radio days, when all antennas were small compared to the wavelength. The boom of mobile phones triggered a second wave of intense research activity on these devices, which continues today where virtually everything has a wireless connection. This intense research activity has produced interesting and usefully results on the physical limitations of such antennas, design rules and optimal designs. Since the beginning of the century, the number of medical, sports, or security applications (to name just some of them) requiring implantable or wearable communication devices has grown at a high speed, launching the interest for wearable or implantable ESAs. Many interesting designs have been published to this date, but we only start understanding the fundamentals of such antennas. Neither physical bounds on their radiation characteristics nor optimal designs or design rules are yet available. In this contribution, I will highlight the main similarities and differences between classic ESAs and antennas for wearables and implants, illustrated by practical examples